The present invention relates to an injection molding machine and in particular to a rapid-action mold closing and clamping arrangement for the molding machine.
Clamping mechanisms based on hydraulic actuation have been know for some time. They are preferred over mechanical toggle mechanisms in most cases because they can accommodate larger opening and closing strokes and impart their clamping force at the center of the moving platen rather than the edges, which promotes platen bending.
The design problems of hydraulic clamps are how to minimize the volume of oil to be moved and how to accommodate a wide range of mold shutheights and opening strokes in the same design. Obviously moving and compressing large volumes of oil requires time and energy to be consumed, both of which are counterproductive.
U.S. Pat. No. 4,443,179 to Wohlrab shows a mechanism that uses a small diameter cylinder to effect the opening and closing strokes of the moving platen, and a large diameter clamping cylinder to effect the clamping force transmitted to the mold. A large valve is used to allow the large volume of oil required in the clamp cylinder to enter rapidly during the closing stroke. The intent in this design is to speed up the filling and emptying of the main clamp cylinder by providing a large opening via the valve.
U.S. Pat. No. 4,605,367 to Gutjahr shows a similar feature. A large clamp cylinder is filled and emptied by means of large valves.
U.S. Pat. No. 4,744,740 to Kojima also shows a similar arrangement to the above without any attempt to minimize the volume of oil flow required to fill the clamp cylinder when the clamp closes.
U.S. Pat. No. 4,846,664 to Hehl shows a different approach to the same problem. The oil in a clamping cylinder is allowed to flow past a clamping piston during the opening and closing strokes which are effected by two outboard cylinders. The clamp piston is sealed to its rod by a sliding sleeve when clamp force is required.
U.S. Pat. No. 4,158,327 to Aoki also shows an arrangement allowing oil to flow past the clamp piston during clamp stroke motions. A valve closes off flow channels in the clamp piston when the clamp force must be developed. A small cylinder moves the main clamp piston through the opening and closing strokes.
Japanese patent number 61-12316 shows another way of doing the same thing. A clamp piston has flow channels which are opened or closed by a valve.
All the above devices have the following disadvantages:
(1) The clamp piston travels the same distance as the clamp closing stroke, subjecting the seals to accelerated wear. Since these seals are also subjected to high pressure clamping forces their service life is comparatively short; PA1 (2) A large volume of oil must fill the clamp cylinder during the closing stroke and be compressed before the full clamp force can be generated. The movement of this large volume of oil is effected by providing large valves and using suction to move the oil rather than pumping. However the time and energy required to compress the oil prior to developing clamp force is considerable; and PA1 (3) Moving large volumes of oil in and out of the clamp cylinder causes the oil to be heated by friction. This is detrimental to the oil which must be cooled using an intercooler. Again another waste of energy.
These disadvantages were largely addressed by a design know as a "shutter" clamp. The original versions of this design are shown in U.S. Pat. No. 4,017,236 to Penkman and U.S. Pat. No. 4,230,442 to Rees. In this design, a small cylinder performs the opening and closing stroke function. When the platen is in the mold closed position, "shutters" are moved to block the column and effectively transmit the clamp force developed by a set of pistons to the column and hence the moving platen and the mold. To open the clamp, the hydraulic pressure on the clamp pistons is released, the shutters are retracted and the small cylinder pulls the moving platen open. The column passes inside the hollow clamping pistons as it retracts, thus reducing the length of the overall assembly.
The advantages of this design is that only small volumes of oil are moved and pressurized and that the clamp piston seals travel a very short distance on each cycle. This reliable, fast acting, energy efficient, hydraulic clamp was further developed by incorporating the clamp stroking cylinder inside the clamp arrangement to further shorten the assembly. This is shown in a brochure dated "1981" published by the assignee of the instant application.
The disadvantage of this arrangement is that the time taken for the shutters to move penalizes the clamp speed slightly.
A third approach uses a clamp closing mechanism which grips the tiebars of the machine prior to the clamping force being developed. U.S. Pat. No. 3,183,555 to Siegel illustrates a system having hydraulically activated "gripper" bushings which grip the tiebars when the mold is closed. Each tiebar is fitted with a clamping cylinder which when activated pulls the tiebars and the "gripped" moving platen toward the stationary platen, effectively clamping the mold. Opening and closing of the mold is effected by a piston and cylinder connected to the moving platen. A similar system is shown in French Patent No. 2,174,361 to Guerin.
Japanese Published Patent Appliction No. 1-245,999 to Kenji et al. illustrates a verticle pressing device having a movable element connected at its edges to a plurality of supporting shafts. A clamp mechanism is provided at each corner of the device to ensure a proper degree of compression. Each clamping mechanism has a flexible sleeve for gripping one of the supporting shafts when it is held against the supporting shaft by means of hydraulic pressure. A clamping or compressive force is applied to each shaft by a piston-cylinder arrangement. The flexible sleeve for gripping the shaft forms part of the piston. Position-detecting sensors may be provided to detect the position of the movable frame on each of the supporting shafts. The fluid pressure in piston-cylinder may then be adjusted to maintain the movable frame and the fixed frame in a parallel relationship.
Japanese Patent No. 1-49088 and Japanese Published Patent Application No. 2-172,711 to Nakagawa et al. each illustrate a friction clamp mold-fastening device for a mold. The mold-fastening device comprises a plurality of piston-cylinder arrangements within the four corners of the moving platen. Each double-acting piston has an inner wall formed by a flexible sleeve for gripping a tiebar along which the platen moves. A clamping force is applied to the mold in a closed position, after the tierods have been gripped by the respective flexible sleeves, by applying a fluid pressure within the cylinder which causes the piston to move toward the fixed platen. The flexible sleeve in the Nakagawa et al. application has a threaded portion which joins it to the supporting shaft portion of the piston.
The principal disadvantage of these arrangements has been that the clamping force is delivered to the corners of the moving platen. The application of the clamping force at the corner causes the platen to bend, particularly if the clamping force at one corner is greater than that applied at another corner. The result is that the mold is unevenly clamped.
Still further, these prior art systems grip the tiebars when they are in a relaxed state and then apply a clamping force which causes stretching of the tiebars or supporting shafts. This stretching must be physically accommodated. The way it is accommodated is either by the gripper bushing relaxing and allowing the tiebar to slip or by the gripper bushing stretching with the tiebar. In the former situation, the clamping forces being applied through the gripper bushing decreases. In the latter situation, the stretching of the flexible wall of the gripper bushing is resisted by the ends of the piston assembly to which it is attached. Ultimately, the bushing fails entirely as a result of stresses building up in its end portions. For those systems where the flexible wall of the bushing has threads which engage mating threads on a support structure, a fretting action occurs in which the threads are deformed. Ultimately, the distribution of forces over the piston becomes non-uniform, the flexible wall of the bushing wears and corrodes.
A newer version of this principle is shown in U.S. Pat. No. 4,571,169 to Shima. Tiebar clamping cylinders push an intermediate frame against a rotary shutter causing columns to be pushed against a moving platen and clamp the mold against the stationary platen. A limited range of shutheights is available with this machine as the clamping piston has a limited stroke inside the cylinder.
While Shima clamps the platen in its center overcoming the bending problem, it has a time penalty caused by waiting for the shutter to move twice on each cycle. An additional disadvantage is the limited range of mold shutheights. Even in the arrangement shown, the volume of oil to be compressed in each clamping cylinder is becoming large. Increasing the length of the cylinder to allow a larger range of mold shutheights would worsen this disadvantage.